The present invention relates to apparatus and methods for heating and more particularly to systems utilizing high pressure combustion air and high velocity flue gas and still more particularly to a combustion system for boilers producing low NOX and CO in the stack emissions.
Heat exchangers function to remove or add heat from one fluid to another. A common heat exchanger used in industrial applications involves a plurality of parallel tubes with one fluid flowing through the tubes and another fluid flowing across the tubes thereby exchanging heat. Such an exchanger is preferably constructed such that the two fluids are not allowed to mix and that the heat from one fluid is transferred to the other fluid through the walls of the tubes. This type of exchanger is often employed in industrial boilers. In a boiler, a burner combusts a mixture of air and gas to create hot flue gases. These hot gases pass through the boiler and across the tubes, sometimes called water tubes. Heat is transferred from the hot flue gasses across the tube walls and into the fluid that is being heated, usually water (water tube boilers) or some type of crude oil or derivates thereof (petro-chemical boilers). The hot flue gases pass through the exhaust stack and are emitted into the atmosphere.
Various types of burners are used with boilers. Atmospheric burners may be used but provide the poorest control of the flame, allow only incomplete and uneven combustion of hydrocarbon fuel, normally gas, and also provide poor heat transfer through the tube walls. The deficiencies in transferring heat through the tube walls of water or petro-chemical boilers, is always on the gas side of the tube walls over which the hot flue gases flow.
Stack draft is a problem for a process heat vessel of any kind or description. It causes all sorts of problems from incomplete complete combustion, uneven heat transfers, to a reduction of draft when the units are trying to increase heat input.
Also atmospheric burners and all long flame forced draft (blower air) burners have to have a stack draft to function. Stack drafts draw in air, flame and combustion gases through the boilers on a path of least resistance which results in a wide variation of heat transfer through the tube walls. Normally these stack drafts are in the −3″ to 4″ water column (w.c.) range, i.e., negative pressure caused by a negative partial vacuum.
The controls for prior art burners are located at the inlet side of the burner and are subject to stack draft-negative pressure.
Prior art boilers and petro-chemical heaters also have the problem of soot (coke) build-up on the insulation walls around the tubes. All of the petrochemical units have excessive tube warpage caused by uneven heat and hot spots. This coking, warpage, uneven heat distribution and hot spots also cause internal problems on the fluid side of the tube wall.
The government is highly regulating emissions into the atmosphere from vessels heated by combustion. In particular emissions from exhaust stacks from heat exchangers or boilers are required to have a low content of CO and NOX.
Atmospheric burners have high pressure (psi) gas orifices that entrain atmospheric air into the gas stream in order to get the fire started. Then the exhaust stack draft dominates to draw in the balance of the air passing through the boiler. How much air is drawn in is not known, but it is a lot more than is normally needed for good combustion, i.e., excess air. Regardless of the percentage of excess air used over that required for combustion, the excess air causes the boiler to produce an atrocious amount of CO which is very undesirable.
Prior art burners have no control over the flames in their burners. All of the prior art atmospheric burners and primary forced draft burners with the two valve control system cannot be controllable at any set NOX emission number. For instance, the John Zink Company markets and guarantees a burner that will only produce 7 parts per million (PPM) NOX. The Zink burner will produce such low NOX emissions as long as the controller does not move the valves in the system. If the valves move, the system becomes uncalibrated and will no longer return to the low 7 PPM NOX. This is true of all two-valve systems because they have no controllability. Also if the NOX emissions change, then the CO emissions change and the ratio of air/gas mixture also change.
To lower NOX, prior art burners have attempted to use excess air, which in turn lowers the maximum flame temperature. However, when the prior art burners increase excess air, the NOX emissions come down but the CO emissions increase.
All of the atmospheric burners and the forced draft burners have long blue flames. Blue fires are in the ultra violet light range and therefore are very poor emitters of radiant heat.
Still further the long flames of prior art burners start getting erratic at about 30 PPM NOX and very few will reduce the NOX down to 20 PPM. The flames have been known to get so erratic at sub 30 PPM that the U.V. (ultra-violet) flame scanner will lose sight of the flame and shut down the burner.
Summarizing, the burners presently being used by the boiler industry have elongated flames and attempt to control combustion with a two valve system (one on the air and one on the gas) which is impossible to control on ratio and is not repeatable. These fires are not controllable as they rely on a negative stack draft to operate. They will not operate at positive pressure therefore they will not furnish high velocity forced convection heat transfer. They lack uniformity and by the nature of the long lazy flames, they cause coking because they cannot burn the CO. A negative pressure causes hot spots, uneven heat distribution across tubes, incomplete combustion which leaves CO (unburned fuel) in the exhaust emissions. The CO cools the tubes and causes soot or coke on the tubes. Soot also interferes with heat transfer. A stack draft also loses some of its pulling power at the higher firing rates when things are hotter; this is when more negative draft is required.
The advent of forced draft burners, using combustion air blowers, resulted in a vast improvement in BTU's per hour per cubic foot combustion space. This was made possible by improved air-gas mixing. These forced draft burners, however, still require a stack draft of 2″ to 4″ w.c. and still produce long uncontrolled flames but shorter with faster burns than atmospheric burners. Normally the forced draft burners operate best with 8″ w.c. air pressure or less. They still have no control of their flames.
Area or refractory lined combustion blocks may be required by some firing rates (BTUH per cubic foot) or design requirements.
My prior art U.S. Pat. Nos. 4,309,165; 4,410,308; and 4,556,386, hereby incorporated herein by reference, disclose a diffuser head which is angled to the outside at a lesser angle than a flat surface. Such diffusers are made of low carbon steel that perform quite well, 10 years plus service life. The prior art diffuser head tends toward a blue flame and extends in length at the higher excess air flow rates. These prior art patents have a higher flame temperature and rely upon radiant heat rather than forced convection.
The present invention overcomes the deficiencies of the prior art.